Abstract: Four carbon/carbon composites with densities of 1.75-1.81 g/cm³ were produced with matrix carbons derived from (a) coal tar pitch and furfural acetone resin by impregnation-carbonization, (b) natural gas by isothermal chemical vapor infiltration and (c) xylene by film-boiling chemical vapor infiltration. Their mechanical properties and thermal conductivities (TC) were compared and correlated to the matrix carbon microstructures. Results showed that the strength of the composites decreased as the carbon matrix changed from natural gas pyrocarbon (PyC_N), to resin-derived carbon, to xylene PyC (PyC_X), and to pitch-derived carbon. The highest flexural and interlaminar shear strengths of 208.7 and 26.4 MPa, respectively, were obtained for the PyC_N matrix. The large amount of fiber pull-out and step-like matrix fracture contributed to a high toughness of composites with PyC_N and PyC_X matrices. Relatively low strength and toughness were obtained for the pitch-derived carbon. The change of TC with the matrix type was consistent with the graphitization degree and apparent crystallite height. The highest in-plane and out-of-plane TC reached 148.2 and 75.4 W/(m·K), respectively, for the PyC_X matrix, which was due to its carbon layers having the highest preferred orientation. The chaotic structure and large number of defects of the resin-derived carbon produced a relatively low TC. PyC_N was the most suitable matrix for composites with excellent mechanical properties. PyC_X should be used to improve the TC and toughness of the low-cost composites.